REVIEW OF THE TIMING, EXTENT, AND CLIMATIC SIGNIFICANCE OF WESTERN U.S. MOUNTAIN GLACIATION DURING THE PENULTIMATE (MARINE ISOTOPE STAGE 6) GLACIAL MAXIMUM

Prominent moraine belts corresponding to the growth of western U.S. mountain glaciers during the last two glacial maxima (marine isotope stages (MIS) 6 and 2) are widespread geomorphic features in this region. The MIS 2 deposits and corresponding glacial positions have been extensively mapped, dated, and used to reconstruct paleoclimatic conditions during the last glacial maximum. By comparison, the timing, extent, and climatic significance of the older MIS 6 glaciations in the western U.S. are sparsely documented. Glacial deposits associated with MIS 6 are commonly designated as ‘Bull Lake’ in the Rocky Mountains, ‘Lamoille’ in the Great Basin, and ‘Tahoe’ in the Sierra Nevada, following historical nomenclature. Until recently, relatively few high-resolution age data existed to support the presumed correlation of these glacial stages across the western U.S. and their assignment to MIS 6. Here we review available evidence and present fresh insight on western U.S. glaciations during MIS 6. Multiple dating techniques including obsidian hydration, K-Ar, ²³⁰Th/U, and terrestrial cosmogenic nuclides have previously been applied to deposits marking the penultimate major advances of glaciers in some western U.S. mountain ranges, and support their correlation with MIS 6. Cosmogenic nuclide exposure dating studies suggest moraine deposition in the Sierra Nevada, Great Basin, and Northern Rocky Mountains during the latter half of MIS 6, overlapping with the global ice volume maximum. Geomorphic mapping of dated moraine positions reveals that the relative extents of MIS 6 versus MIS 2 glacial maxima vary dramatically across the western US. Some landscapes exhibit considerably more expansive MIS 6 ice, whereas other localities preserve MIS 2 end moraines that evidently overrode the older MIS 6 moraines. Regional-to-local differences in the relative magnitudes of MIS 6 and MIS 2 glacier extents may reflect some combination of spatially variable climatic influences, ice dynamics, and tectonics. Few attempts have been made to assess paleoclimates in the western U.S. during MIS 6, but one recent modeling study using a coupled mass/energy balance and glacier flow model suggests a temperature depression of about 6.4 ± 1.0°C relative to present in the Colorado Rocky Mountains during MIS 6, assuming no change in precipitation.